This invention relates to the operation of computer controlled machine tools which are capable of performing milling operations on a workpiece. In such machine tools, a programmed spindle speed and programmed feed rate are applied to the machine tool before it begins to cut the workpiece. The programmed spindle speed and feed rate are selected to be suitable for the material of the workpiece, the type of tool and the depth of the cut. However, the depth of the cut is not always uniform but may vary from one location on the workpiece to another, and if the feed rate is held at a constant level, the horsepower developed at the cutter will vary as the cut progresses. At the peak cutter horsepower, the optimum cutting conditions will be exceeded, while at the minimum cutter horsepower, the optimum cutting conditions will not be reached. The principal object of this invention is to provide a method of optimizing the operation of the machine tool under all cutting conditions.
Another problem encountered in such computer controlled machine tools is that the tools eventually wear off their cutting edges and become too dull for use. In the past, the dull tool problem was handled by measuring the total tool cutting time for each tool and then replacing each tool after a predetermined cutting time had elapsed. However, the cutting life of a tool varies in accordance with the conditions of each cutting operation and cannot be accurately calculated in advance. If an unusually large number of deep cuts are encountered in the workpieces, or if cutting conditions are improper, the tool will become dull before the end of its pre-calculated lifetime, while ideal cutting conditions may leave the tool sharp enough for further use after the end of its pre-calculated lifetime. Another object of this invention is to provide a method of determining when the cutting tool is too dull for further use and also for determining when the tool is broken.
A further problem involves increasing the feed rate when an air gap is encountered in the workpiece to provide a fast feed rate through the air gap and then reducing the feed rate at the end of the air gap. This has been done in the past by monitoring the torque of the spindle motor, increasing the feed rate when the torque drops below a predetermined value, and then reducing the feed rate when the torque rises above a predetermined value. However, due to the normal variations in torque that result from variations in cutting conditions, it is possible for the feed rate to jump back and forth between fast feed rate and normal feed rate. A further object of this invention is to provide a method of preventing the feed rate from jumping back and forth between the fast feed rate and the normal feed rate. Other objects and advantages of the invention will be apparent from the detailed description herein.